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Adobe patched 23 security vulnerabilities in its Reader PDF viewer on Tuesday, most of them critical, including one that has been exploited by hackers for at least a month or possibly much longer.

In September, Adobe promised to speed up the delivery of today's patches, which were originally meant to ship next week, because attackers were already leveraging a bug in Reader's and Acrobat's font parsing.

Tuesday's fixes updates Reader and Acrobat to versions 9.4 and 8.2.5.

"Adobe is hitting customers with a double whammy today," Andrew Storms, director of security operations at nCircle Security, said via e-mail. "Adobe products continue to be at the top of the target list for malware writers."

"They patched a zero-day flaw in Flash in late September, and today they are releasing their quarterly Acrobat update ahead of schedule because of another zero-day," Storms said.

Tuesday's Reader and Acrobat updates also included a patch released more than two weeks ago for Flash , Adobe's media player. Both Reader and Acrobat include code to run Flash embedded in PDF documents.

Of the 23 bugs Adobe patched, the most notable was the one revealed Sept. 7 by Mila Parkour, an independent security researcher who reported the attack after discovering rigged PDFs attached to e-mail messages.

The vulnerability and attacks received the label "David Leadbetter" after the renowned golf swing coach whose name was used in the subject line of many of those e-mails.

The Leadbetter exploit was called "scary," "clever" and "impressive" by various security researchers in September, in part because it bypassed important defensive measures that Microsoft has built into Windows, ASLR (address space layout randomization) and DEP (data execution prevention).

Most of the attacks using the Leadbetter exploit were "targeted" -- aimed at specific individuals or companies -- rather than used in massive campaigns.

The exploit also relied on a stolen digital certificate to sign some of its files, another hint at a greater-than-average level of sophistication. Chet Wisniewski, a senior security adviser at security software vendor Sophos, compared the exploit to the Stuxnet worm, which also used pilfered certificates.

Wisniewski also noted that the Leadbetter exploit's stolen certificate had signed one component of the malware in 2009, a clue that the attack code, or at least part of it, had been circulating since then.

Of the 23 vulnerabilities patched today, 20, or 87% of the total, were tagged with the phrase "could lead to code execution" by Adobe in its accompanying bulletin .

Unlike some vendors, such as Microsoft, Adobe does not assign threat ratings to bugs in its products, but "code execution" means that attackers could exploit the flaws to hijack the computer.

Microsoft assigns the "critical" label to most, though not all, of its vulnerabilities that allow code execution.

Two of the remaining 23 bugs could be used to crash Reader or Acrobat, while the final vulnerability was a Linux -only issue that could result in an attacker gaining higher levels of access to a machine.

Nine of the 23 bugs were reported to Adobe by security engineers who work for Google -- Tavis Ormandy was credited with eight of those -- while three others were handed to Adobe by HP TippingPoint's Zero Day Initiative bug bounty program.

Also today, Adobe kicked off a series of technical blog posts about the "sandboxing" technology the company intends to add to the Windows version of Reader sometime this year.

Called "Protected Mode" by Adobe, the technology is designed to isolate processes from one another and the rest of the machine, preventing or hindering malware from escaping an application to wreak havoc or infect the computer.

Microsoft uses a similar technology in its Internet Explorer 7 (IE7), IE8 and IE9 browsers on Windows Vista and Windows 7 ; Google also relies on sandboxing to stymie attacks directed at or through the Chrome browser.

Adobe also announced that it will demonstrate Protected Mode later this month at its Adobe MAX 2010, which is slated to run Oct. 23-27 in Los Angeles.

A company spokeswoman said that the MAX 2010 session had been planned some time ago, and cautioned against reading anything into its appearance on the schedule today. She also declined to name the release date for the debut of sandboxing in Reader.

"The sandbox feature that Adobe has promised can't come a minute too soon," Storms said. "We have to hope Adobe has more strategic security initiatives up their collective sleeves, because right now they are struggling just to keep up with attackers."